PROJECT ABSTRACT Acute exacerbations of COPD (AECOPD) contribute to the high morbidity and mortality of the condition. COPD exacerbations are costly, associated with increased healthcare utilization, and contribute to mortality in COPD. The antimicrobial peptide cathelicidin functions in the innate immune system by direct antimicrobial killing, immune cell signaling, and neutralization of lipopolysaccharide. In cohorts of stable COPD patients and smokers at-risk for COPD, cathelicidin levels are associated with lower lung function over time, increased incidence of pneumonia, and an increase in episodes of COPD exacerbations that require change in regular therapy. During an exacerbation of COPD, plasma cathelicidin levels increase, likely due to an acute inflammatory state. To date, there have been no investigations into the trend of plasma cathelicidin levels in AECOPD or the prognostic implications of plasma cathelicidin during hospitalization for AECOPD. This proposal will seek to define cathelicidin level and trend during hospitalization for COPD exacerbation and determine prognostic implication of cathelicidin levels during hospitalization for COPD exacerbation, including need for escalation of care and readmission rates. Successful completion of this proposal will determine the role of plasma cathelicidin levels in identifying high-risk cases of AECOPD.